1. Field of the Invention
The present invention relates to an optical semiconductor device having a light-emitting element and a method for manufacturing the optical semiconductor device.
2. Description of the Related Art
Optical semiconductor devices are used widely as light sources of various device, such as lighting and a display device, in different areas. As the optical semiconductor devices, proposed is an optical semiconductor device which produces white light and the like by combining light emitted by a light-emitting element and light emitted by a phosphor which is excited by the light emitted by a light-emitting element (refer to JP-A No. 2005-311136 (KOKAI), for example).
Such an optical semiconductor device includes a light-emitting element, such as a light-emitting diode (LED), and a base substrate which has a cup-shaped concave portion housing the light-emitting element. The concave portion is provided with a translucent member which seals the light-emitting element. Incidentally, the base substrate is molded by mold resin. In addition, the translucent member is formed of a translucent resin material mixed with a particulate phosphor.
In such an optical semiconductor device, a part of light emitted from the light-emitting element passes through the translucent member, and is then released. Another part is reflected by a side (a side wall) of the concave portion and is then released. Here, the light incident upon the side surface of the concave portion is reflected or absorbed by the base substrate. Incidentally, the reflectance of light depends on the wavelength of incident light, and generally, the shorter the wavelength is, the lower the reflectance is.
Here, a reflective filler such as titanium dioxide (TiO2) is mixed in mold resin of the base substrate in order to improve the reflectance. The reflectance in an ultraviolet region is less than 10% in this case. To improve the reflectance in the ultraviolet region as well, proposed is an optical semiconductor device in which a part of the resin of the base substrate, that is, only the side surface of the concave portion, is plated with silver. Furthermore, a ceramic substrate which is hardly degraded by light is used as the base substrate in order to prevent the base substrate from being deteriorated by light. In this case, the heat dissipation of the light-emitting element is taken into consideration, and the base substrate made of aluminum nitride is thus used as the ceramic base substrate.
However, the partial plating of the resin of the base substrate with silver increases the cost, because the plating requires a complicated manufacturing step such as masking. Moreover, when the ceramic base substrate made of aluminum nitride or the like is used, the cost increases because the ceramic substrate is expensive.
In addition, if the light energy of the light-emitting element increases in accordance with an improvement in the luminous efficiency of the light-emitting element, an increase in driving current and the like, the base substrate is more likely to degraded by light. Since the reflectance of light further decreases due to the degradation of the base substrate by light, the amount of light extracted and the brightness decrease.